x86: Add ENDBR at function entries

x86: Add ENDBR at function entries

Intel Control-flow Enforcement Technology (CET):

https://software.intel.com/en-us/articles/intel-sdm

contains shadow stack (SHSTK) and indirect branch tracking (IBT).
When IBT is enabled, all indirect branch targets must start with
ENDBR instruction which is a NOP on non-CET processors.

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2538
Acked-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Ben Widawsky <ben.widawsky@intel.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3865>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3865>